Plastic lenses have found increasing commercial utility as eyeglass lenses, camera lenses and other optical lenses in recent years, since they are lighter in weight and less fragile in comparison with inorganic glass lenses. As a resin which is currently used in a large volume for the above application, there is a resin obtained by the casting-polymerization of diethylene glycol bisallylcarbonate (hereinafter called "CR-39"). However, although the above resin has many excellent optical characteristics, the refractive index (n.sub.d) of the resin is 1.50, which is smaller compared with those of inorganic glass lenses (n.sub.d =about 1.52). As resins having high refractive indexes, there have already been known polycarbonate (n.sub.d =1.58-1.59), polystyrene (n.sub.d =1.58-1.60), etc. These resins are each a two-dimensional polymer structurally and thermoplastic. They are thus unsuitable for casting-polymerization method which is suitable for production of articles in various models such as fabrication of eyeglass lenses, and their post-molding processings, especially, their rough-grinding and smoothing (hereinafter merely referred to as "grinding") work is difficult, because it make diamond-made grindstones loaded. Therefore, use cf these resins are presently limited to some sort of safety eyeglasses and the like which do not need grinding work.
Accordingly, there is a strong desire for the development of a lens-making resin which has a refractive index higher than that of the lens-making resin prepared by polymerizing CR-39, has an excellent post-molding processability similar to CR-39 owing to its three-dimensional crosslinking structure when polymerized. A variety of researches has already been carried out with a view toward developing a resin which would meet the above-mentioned desire, resulting in proposals of resins obtained by copolymerizing CR-39 and second monomers having refractive indexes higher than that of CR-39 when measured as their respective homopolymers (see, Japanese Patent Laid-open Nos. 79353/1976, 7787/1978, 77686/1979, 15118/1980 and 36601/1981). The refractive indexes of the thus-copolymerized resins are however inherently limited because they employ CR-39 as their principal components. It was thus difficult to obtain a resin having a high refractive index, for example, a refractive index of 1.55 or higher.
In order to obtain a resin having a still higher refractive index, it is urged to use a bifunctional monomer which can afford a homopolymer having a refractive index higher than that of CR-39. However, each of bifunctional monomers which have been proposed to date resulted in a polymer having impact resistance much poorer compared with the homopolymer of CR-39 when polymerized singly. Thus, some attempts have been made to improve the impact resistance of these bifunctional monomers by copolymerizing them with a unifunctional monomer. Here, each matching unifunctional monomer is required to have a high refractive index when measured as its homopolymer if one wants to obtain a copolymer having a high refractive index. For this reason, styrene or a halogen-substituted styrene is presently used as such a unifunctional monomer. However, use of bifunctional monomers different from CR-39, which have heretofore been proposed, in combination with the styrenes as unifunctional monomers is accompanied by such drawbacks that it tends to result in development of polymerization strain and is difficult to obtain polymers having uniform refractivity distribution because there are considerable differences in polymerization reactivity between such bifunctional monomers and the styrenes and the proportions of the bifunctional monomers and the styrenes cannot be varied freely due to poor miscibility therebetween.
With the foregoing in view, the present inventors carried out an extensive research with a view toward making improvements to the above-described drawbacks. As a result, it has been found that a resin, which has a high refractive index and excellent processability such as grinding processability and superb impact resistance, exhibits excellent miscibility between its starting unifunctional monomer and bifunctional monomer upon copolymerization thereof, is less susceptible of developing polymerization strain and is thus suitable for use in the production of high-refractivity lenses, can be obtained by copolymerizing a specific unifunctional monomer and a bifunctional monomer, leading to completion of this invention.